Good question. I don't have any idea what others would consider proper, but I estimate the percentage of the receiver that still has any color whatsoever, and then of that estimate the intensity compared to what the original color might have been. A much handled field gun might score 10/10, whereas a frequently used but carefully handled gun might score 90/80. Works for my use, but almost certainly too complicated for wide application, and would always subject to the vagaries of interpretation.